Langendorff hanging heart preparations (isolated, perfused, functioning rat hearts) have been shown to suffer functional damage under conditions of ischemia. Various treatments have been imposed upon the hearts to minimize the functional damage during such ischemic periods. in order to assess the biochemical changes occurring during these ischemic periods, the heart tissue has been extracted and the extracts have been examined for enzymic activities and for carbonyl content. Conditions of anoxia, which are serious enough to provide only a 50 percent recovery of function, cause little change in the creatine kinase (CPK) activity, whether during the period of anoxia or during subsequent reperfusion. At the same time, however, lactate dehydrogenase activities rise during the anoxia and return to initial values upon reperfusion. Carbonyl content, a measure of oxidative damage, undergoes a modest increase during anoxia but rises dramatically upon reperfusion, consistent with the idea that the free radical production and associated oxidation occurs upon the reintroduction of oxygen to the anoxic tissue, During the period of anoxia, there is a marked loss of ATP accompanied by rises in AMP and its breakdown products, inosine, hypoxanthine, and xanthine. Upon reperfusion, the ATP levels rise to near initial values and the AMP, xanthine, and hypoxanthine levels return to their original low levels. Interestingly, ADP levels change very little during the periods of ischemia or reperfusion.